As the coverage area of the Long Term Evolution (LTE) network is growing constantly, and the LTE technologies are developing constantly, a demand for a capacity may be difficult to satisfy in the traditional scheme of coverage by a macro cell due to an increasing number of subscribers, and a growing demand for a bandwidth, so that a demand of subscribers for traffic may not be satisfied simply in the simple macro coverage scheme particularly in some densely populated region where there may be a blind area in the homogenous coverage scheme, thus discouraging the subscribers from an access thereto. In view of this, the heterogeneous network has been introduced in LTE so that the LTE network can be deployed in a more flexibly pattern, and also the concept of a small cell has been proposed in that the small cell typically has a smaller coverage area and lower transmit power, and the small cell can be deployed at a shorter distance from the subscribers, e.g., indoors, to thereby improve the data rate of the subscribers.
There are significantly improved required peak rates of the Long Term Evolution-Advanced (LTE-A) system up to 1 Gbps in the downlink and 500 Mbps in the uplink as compared with the LTE system. The LTE-A system is also required to be well compatible with the LTE system. Carrier Aggregation (CA) has been introduced to the LTE-A system to thereby accommodate the improved peak rates, compatibility with the LTE system, and full use of spectrum resources as needed.
Carrier aggregation refers to such a mechanism that a plurality of cells can be aggregated for a User Equipment (UE) (also referred to a terminal), and can provide the UE concurrently with a data transmission service. Carriers corresponding to the respective cells in the system with carrier aggregation can be consecutive or inconsecutive in frequency, and there is a bandwidth up to 20 MHz of a carrier of each cell for compatibility with the LTE system, where there may be the same bandwidth or different bandwidths of the respective component carriers.
With carrier aggregation, the cells over which the user equipment operates include one Primary Cell (PCell) and several Secondary Cells (SCells), where the primary cell is responsible for the majority of control and signaling, for example, transmitting an A/N feedback for downlink data, a Channel State Information (CSI) report, a Sounding Reference Signal (SRS), and a Dedicated Scheduling Request (D-SR), making a Contention Based Random Access (CBRA), etc., where a Physical Uplink Control Channel (PUCCH) is only active over the PCell; and the secondary cells are primarily responsible for acting as resources over data are transmitted, where a Physical Uplink Shared Channel (PUSCH) and an SRS can be transmitted in the uplink.
At present, the UE can not be configured with a set of serving cells beyond a capacity of aggregated carriers for the UE, that is, the UE for which no uplink carriers can be aggregated can only operate over the PCell in the uplink, that is, an uplink transmission primary carrier can only be active over the PCell.
In the scenario of carrier aggregation, in order for the UE to save its power, the activation/deactivation mechanism has been introduced. In the LTE-A system, a cell is activated or deactivated, particularly explicitly or implicitly, without distinguishing between the uplink and the downlink. It shall be noted that the cell can be deactivated only implicitly.
In the explicit activation mechanism, the base station transmits an activation or deactivation Media Access Control-Control Element (MAC CE) to control the active state of the aggregated cells of the UE. FIG. 1 illustrates the format of the activation or deactivation MAC CE, where the length of the activation or deactivation MAC CE is 8 bits including C7, C6, C5, C4, C3, C2, C1, and R respectively, each of which corresponds to a cell, that is, the last bit corresponds to the PCell, where since the PCell will never be deactivated, the R bit is defaulted as 0; and the other bits correspond respectively to respective SCells, that is, Ci corresponds to the SCell numbered i. If Ci is “i=1”, then it will indicate that the SCell is activated, and if Ci is “i=0”, then it will indicate that the SCell is deactivated.
In the implicit deactivation mechanism, the cell is deactivated implicitly by introducing a deactivation timer configured per UE, and maintained per cell. If the base station does not configure the timer, then the length of the timer will be defaulted as the infinity.
The deactivation timer is maintained per cell as follows: if the UE receives activation signaling for some cell, then the UE will start/restart the deactivation timer corresponding to the cell; and while the timer is operating, once the UE receives scheduling signaling for uplink or downlink transmission over the cell, then the UE will restart the deactivation timer of the cell. It shall be noted that if there is across-carrier scheduling, then both the deactivation timers of the scheduling and scheduled cells will be started, where the scheduling signaling for uplink or downlink data transmission will be carried over a Physical Control Channel (PDCCH); and if the deactivation timer expires, then the UE will deactivate the cell.
In summary, there are base stations of a macro cell and small cells in the scenario of heterogeneous network deployment. Since the distance between the UE, and a transmitter of the macro cell is typically longer than the distance between the UE, and a transmitter in a network of the small cells, if carriers are aggregated for transmission by the UE, then if the macro cell is configured as a PCell of the UE, then an uplink transmission carrier of the UE will be transmitted only over the PCell at high transmit power, thus resulting in significant interference to another UE; and if a small cell is configured as a PCell of the UE, then since there is such a small coverage area of the small cell that a cell handover has to be made frequently while the UE is moving, thus interrupting service transmission during the handover, which may significantly degrade the performance, and also increase a signaling load in a core network. In the prior art, the cell handover is made as a handover among the cells for both the uplink transmission carrier and a downlink transmission carrier of the UE instead of a handover among the cells for only the uplink transmission carrier of the UE, thus failing to consider both the transmission advantages of the different cells.